The invention relates to a switching power amplifier (SPA) having an extremely high efficiency, e.g. for high power audio amplification. The SPA operates at high switching frequency and includes a switching power supply (SPS) with line isolation.
Power amplifiers are devices designed to amplify an input signal and provide an undistorted output signal which is independent of supply voltages, load fluctuations over frequency, operating temperature, etc. Specifically, SPAs provide significantly higher efficiency by employing high speed switches.
The disadvantages of conventional SPAs are many. The audio signal is usually converted into a specific pulse width modulated signal (PWM) which is a combination of analog and digital signals. The pulse width can have any value within a switching period and is influenced by all kinds of errors. Each of the complementary output transistors must switch a least once within the high frequency period. A poor tracking of the transistor switching times results in asymmetric errors in pulse width. Power supply ripples and pulse amplitude errors depend on output power which increases distortion level, particularly near crossover point. Pulses are lost near extremes of modulation. A dead time of the transistors is inevitable in order to prevent overlapping of the conduction phases thereof due to drive asymmetry, poor transistor recovery characteristics or inadequate deadband of a control circuitry.
Furthermore, the output filter in resonant configuration is designed for one specific frequency. Its performance is poor due to regulation concepts, such as the PWM, rather than practical limitations of the components. A very high switching frequency is necessary, mostly tens of times higher than a maximum frequency of the amplified signal. This results in a further reduced efficiency and a relatively high distortion level due to the limited switching performance of the power switches. Another unsolved problem is a desirable load having a relatively stable impedance over frequency. The equivalent switching frequency of the individual transistors is actually significantly higher as the switching times thereof are a fraction of the switching period. An SPS for stabilizing supply voltages is common. This further increases the complexity and in most cases generates a huge amount of EMI/RFI, especially into the line. Numerous interference suppressors and protection circuits ar inevitable.
In conclusion, except for a small size and higher efficiency of a common SPA there is no reason for a user to give preference to it over a conventional analog power amplifier.